Musnad Ahmad 27:832

'Abd al-Rahman narrated to us, Sufyan narrated to us, from 'Abdullah ibn Dinar, who said: I was with Ibn 'Umar, I and another man, then he called another man. Then he said: "Move apart, for the Messenger of Allah (peace and blessings of Allah be upon him) forbade that two people should whisper privately to the exclusion of a third."